Shock timing lowers transvenous defibrillation energy requirement

Previous studies suggested that time periods exist during ventricular fibri llation when defibrillation shocks are more effective. However, there is no agreement on the amount of energy that can be saved or whether an implanta ble defibrillator can time shocks to these time periods. We conducted a stu dy having two parts to investigate if there was any advantage to synchroniz ing internal defibrillation shocks to morphological patterns in ventricular fibrillation (VF). VF electrograms were recorded from the same three-elect rode lead system used for internal defibrillation. In Part 1, we found no d ifference in the probability of successful defibrillation between shocks th at were delivered into coarse and fine VP (48% vs 46%). However, shocks tha t were delivered to the upslope of coarse VF electrograms were more efficac ious than those to the downslope of the waveform (67% vs 39%, P < .001). In the second study, we developed a real time computer system to prospectivel y deliver shocks on the upslope feature we identified in the first study. W e found that the energy requirements at E-50 and E-80 were significantly lo wer for shocks delivered on the upslope of coarse VF than those delivered r andomly at the end of 10 sec. We estimated a probability of success (POS) d efibrillation curve using a maximum likelihood method for the timed and ran dom shocks. The POS curve width was significantly narrower for shocks that were delivered to the upslope feature than the control treatment (7.1 +/- 0 .9 vs. 10.8 +/- 1.7 J, P < 0.01). If these findings extend to clinical defi brillation, they may allow programming of internal defibrillators at lower energies. This could reduce potential postshock cardiac dysfunction, allow production of smaller devices, and improve battery Life.